Spiral wound gas filtration module with specific adhesive

ABSTRACT

A membrane envelope comprising a feed spacer, one or more membrane sheets and an adhesive, to give a glue line laminate having a tensile E-modulus of at least 1600 N/mm 2  and/or an elongation at break of 13% or less.

RELATED APPLICATION DATA

This application is a National Stage Application under 35 U.S.C. 371 ofco-pending PCT application number PCT/GB2014/052948 designating theUnited States and filed Sep. 30, 2014; which claims the benefit of GBapplication number 1317528.6 and filed Oct. 3, 2013 each of which arehereby incorporated by reference in their entireties.

This invention relates to membrane envelope stacks, to spiral wound gasfiltration modules and to methods for their preparation and use for theseparation of gases.

Spiral wound gas separation modules typically comprise one or moremembrane envelopes, each envelope comprising a feed spacer sandwichedbetween two membrane sheets, or one membrane sheet folded around thefeed spacer. The outside edges of the membrane sheets are typicallysealed on all but two sides, e.g. using an adhesive seam, to provide themembrane envelope having two, opposite open ends. The open ends of eachmembrane envelope are then glued to the open ends of the next membraneenvelope, creating a gas-tight seal between adjacent membrane envelopeswhile also ensuring that the open ends remains open so that feed gas maypass along the inside of the membrane envelope but not between themembrane envelopes. The region where the adhesive bonds the membraneenvelopes together is referred to as a glue line and in this area themembranes and adhesive together form what is called a glue linelaminate. Typically a permeate carrier is included between each membraneenvelope and the next to keep them apart, thereby allowing any gas whichpermeates through the membrane to flow along the permeate carrier andinto a central permeate collection tube. In some cases the glue linelaminate consists of two membranes (from adjacent membrane envelopes)and adhesive. In other cases, the permeate carrier extends beyond theglue line and therefore the glue line laminate consists of two membranes(from adjacent membrane envelopes), the permeate carrier and theadhesive which bonds the membranes and permeate carrier together in agas-tight manner.

Gas separation modules can be constructed by winding a stack of membraneenvelopes around a perforated, central permeate collection tube,Optionally a permeate carrier is included between adjacent membraneenvelopes, In use, the gas to be separated (or filtered) is introducedunder pressure at one end face of the module and is allowed to travelaxially along the module through the feed spacers inside the membraneenvelopes. Because edges of adjacent membrane envelopes are adheredtogether with a gas-tight seal, the feed gas cannot enter the areabetween the membrane envelopes (typically containing a permeate carrier)without first passing through the membrane wall of the membraneenvelope. As the feed gas flows axially through the module, along thefeed spacer, the gas which permeates through the membrane sheet flowsthrough the area between the membrane envelopes (e.g. through thepermeate carrier) and to the permeate collection tube. Retentate whichhas not passed through the membranes is removed from the far end of themodule. Permeate is removed through the far end of the permeatecollection tube.

One of the problems with membrane envelope stacks comprising membraneenvelopes adhered together using an adhesive seam is the tendency of theseams (i.e. glue line laminates) to form undesirable blisters. Theblisters typically appear after the first time the module containing themembrane envelope is used, often when the pressure of the gas feed isreduced. While not wishing to be bound by any theory, the presentinventors believe that when a membrane envelope comprising an adhesiveis being used, the adhesive may absorb gas molecules when under highpressure. Then, when the pressure is reduced after use, the molecules ofgas absorbed into the adhesive may expand to form blisters or otherdefects in the adhesive seal. The occurrence of blisters is undesirable.The blisters can restrict or block the gas feed flow, the adhesive maycrack or fail, or flake-off and thereby restrict the flow of gas.Blisters can also lead to gas leaks, reducing the selectivity of the gasseparation membrane modules.

U.S. Pat. No. 5,034,126 ('126) describes a method for preparing a spiralwound membrane which comprises adhesive seals. As noted in '126, theseals may burst under high pressure when the module is in use. Howeverthis technical problem of bursting in use is different from the problemaddressed by the present invention where blistering occurs afterpressure applied to the membrane is reduced.

U.S. Pat. No. 5,041,517 ('517) describes polyurethane adhesives andtheir use for bonding laminates of permeable membranes supported onnylon fabrics. According to '517, column 1, line 63, it is essentialthat the adhesive remains flexible when cured.

The present inventors have found that one may reduce the likelihood ofblistering by using membrane envelopes as per the present invention.

According to a first aspect of the present invention there is provided amembrane envelope stack comprising membrane envelopes bonded together bymeans of an adhesive to give a glue line laminate having a tensileE-modulus of at least 1600 N/mm² and/or an elongation at break of 13% orless.

A second aspect of the present invention provides a spiral wound gasfiltration module comprising a membrane envelope stack according to thefirst aspect of the present invention.

The term “comprising” is to be interpreted as specifying the presence ofthe stated parts, steps or components, but does not exclude the presenceof one or more additional parts, steps or components.

Reference to an element by the indefinite article “a” or “an” does notexclude the possibility that more than one of the element(s) is present,unless the context clearly requires that there be one and only one ofthe elements. The indefinite article “a” or “an” thus usually means “atleast one”.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates how membrane stack and module according to thepresent invention may be prepared.

FIG. 2 is a partially exploded, perspective view of a gas filtrationmodule according to the present invention.

FIG. 3 shows the dimensions of a Specimen Type 5 test strip to be cutfrom the glue line laminate in order to measure tensile E-modulus andelongation at break by the method of ISO 527-3:1995.

In FIG. 1, a permeate carrier (24) is attached to permeate collectiontube (12) having perforations (14). A stack of alternate membraneenvelopes (26) and permeate carriers (24) are aligned on the permeatecollection tube (12). The membrane envelopes (26) comprise a rectangularmembrane sheet (18) folded around a feed spacer (16) and the folded edgeof the membrane envelope abuts the permeate collection tube (12). Thestack is then wound around the permeate collection tube (12) to providea membrane structure comprising two parallel end faces and a third faceof circular cross-section. Adjacent membrane envelopes (26) are adheredtogether such that feed gas passing from the left to the right in FIG. 1can pass along the feed carriers (16) but cannot enter the permeatecarriers (24) without first passing through the walls of membranes (18).In the embodiment shown in FIG. 1, the permeate carrier consists of onepermeate spacer sheet. In a preferred embodiment (not shown), each ofthe permeate carriers (24) comprise two permeate spacer layers and agas-impermeable sheet, wherein the gas-impermeable sheet is locatedbetween the two permeate spacer layers. The two permeate spacer layersmay be provided by, for example, two macroporous sheets (one on eachside of the gas impermeable sheet) or by one macroporous sheet foldedaround the gas impermeable sheet.

Feed gas may be prevented from entering the permeate carriers (24)without first passing through the membranes (18) by depositing anadhesive along the left and right outside edges of the membraneenvelopes (26) to form a glue-line, thereby forming a gas-tight seal.

Referring to FIG. 2, a spiral wound gas filtration module according tothe present invention is designated generally by the numeral (10). Themodule has a central permeate collection tube (12) having perforations(14) along its length. Membrane envelope (26) is wound about thepermeate collection tube (12). Each membrane envelope is oriented topresent an edge generally adjacent the tube (12), a pair of side edgesand an axial edge distal from the tube and oriented to be in parallelwith the axis of the tube. A liquid adhesive (36) is provided alongthree of the outer sides of each the membrane envelope (26) in order toprovide a glue line laminate providing a gas-tight seal between eachmembrane envelope and the next. The fourth outer side of each membraneenvelope (26) is left open (i.e. no liquid adhesive is applied) for gascommunication with the permeate collection tube (12). Permeate carriers(24), membrane sheets (18), and feed spacers (16) are thus spirallywound around permeate collection tube (12) with permeate carriers (24)located adjacent to permeate collection tube (12) and in gascommunication therewith. Referring to the series of layers of membranesheet (18), feed spacer (16) and a second membrane sheet (18) as amembrane envelope (26), typically a stack of membrane envelopes (26) arespirally wound about the permeate collection tube (12) with a permeatecarrier (24) located between adjacent membrane envelopes. The module mayoptionally be formed without permeate carriers (24). During preparationof the modules, the adhesive (36) may be partially cured before thevarious layers are wound onto the permeate collection tube (12) and thenheated after winding to further cure the adhesive (36).

The glue line laminate always comprises two membranes (from adjacentmembrane envelopes) and adhesive. In some cases, the permeate carrier ispresent between the membranes in the region where the glue line isapplied. Therefore the glue line laminate optionally further comprises apermeate carrier.

FIG. 3 shows a the dimensions of a Specimen Type 5 test strip to be cutfrom the glue line laminate in order to measure tensile E-modulus andelongation at break by the method of ISO 527-3:1995.

The height (h) is ignored because the height will correspond to whateverthe thickness is of the glue line laminate under test. The test stripcomprises rectangular end portions at each end and a relatively longcentral portion. The rectangular end portions are gripped in the jaws ofthe tensile stress measuring machine and are slowly pulled apart at aspeed of 30 millimeters/minute, stretching the central portion until itbreaks. The tensile stress measuring machine provides the results forthe tensile E-modulus and elongation at break.

The membrane envelope stacks comprise at least two membrane envelopes,e.g. from 2 to 30, preferably from 10 to 25 membrane envelopes.

The function of the permeate collection tube is to collect the permeategas which has passed through the membranes. Thus the membrane envelopesand permeate carriers are arranged such that the permeate can flowthrough perforations in the permeate collection tube and the retentatecannot reach the permeate collection tube perforations because it hasnot passed through the membranes.

The perforations along the length of the permeate collection tube allowpermeate gas to flow from the exterior of the tube to the interior.Surrounding the permeate collection tube and in gas communicationtherewith there is typically is a permeate carrier. The permeate carrierprovides a gap between the membrane envelopes through which permeate gasmay flow.

The permeate collection tube is typically constructed of a rigidmaterial, for example a metal (e.g. stainless steel) or a plastic.

Preferably the membrane envelopes comprise a feed spacer and one or moremembrane sheets, wherein the feed spacer is sandwiched between themembrane sheet(s).

Typically the membrane sheets are composite membranes, e.g. comprising adiscriminating layer and a porous support. The function of thediscriminating layer is to preferentially discriminate between gases,separating a feed gas mixture into a permeate which passes through themembrane (from the inside of the membrane envelope to the outside of themembrane envelope) and a retentate which does not pass through themembrane. The permeate and retentate typically comprise the same gasesas the feed gas mixture, but one is enriched in at least one of thegases present in the feed gas and the other is depleted in that samegas.

The porous support is typically open pored, relative to thediscriminating layer. The porous support may be, for example, amicroporous organic or inorganic membrane, or a woven or non-wovenfabric. The porous support may be constructed from any suitablematerial. Examples of such materials include polysulfones,polyethersulfones, polyimides, polyetherimides, polyamides,polyamideimides, polyacrylonitrile, polycarbonates, polyesters,polyacrylates, cellulose acetate, polyethylene, polypropylene,polyvinylidenefluoride, polytetrafluoroethylene, poly(4-methyl1-pentene) and especially polyacrylonitrile.

One may use, for example, a commercially available, porous sheetmaterial as the support. Alternatively one may prepare the poroussupport using techniques generally known in the art for the preparationof microporous materials. In one embodiment one may prepare a porous,non-discriminatory support by curing curable components, then applyingfurther curable components to the formed porous support and curing suchcomponents thereby forming the layer of cured polymer and thediscriminating layer on the already cured porous support.

One may also use a porous support which has been subjected to a coronadischarge treatment, glow discharge treatment, flame treatment,ultraviolet light irradiation treatment or the like, e.g. for thepurpose of improving its wettability and/or adhesiveness.

The porous support preferably has an average pore size of at least about50% greater than the average pore size of the discriminating layer, morepreferably at least about 100% greater, especially at least about 200%greater, particularly at least about 1000% greater than the average poresize of the discriminating layer.

The pores passing through the porous support typically have an averagediameter of 0.001 to 10 μm, preferably 0.01 to 1 μm (i.e. before theporous support has been converted into a composite membrane). The poresat the surface of the porous support will typically have a diameter of0.001 to 0.1 μm, preferably 0.005 to 0.05 μm. The pore diameter may bedetermined by, for example, viewing the surface of the porous support byscanning electron microscopy (“SEM”) or by cutting through the supportand measuring the diameter of the pores within the porous support, againby SEM.

The porosity at the surface of the porous support may also be expressedas a % porosity, i.e.

${\%\mspace{14mu}{porosity}} = {100\% \times \frac{\left( {{area}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{surface}\mspace{14mu}{which}\mspace{14mu}{is}\mspace{14mu}{missing}\mspace{14mu}{due}\mspace{14mu}{to}{\mspace{11mu}\;}{pores}} \right)}{\left( {{total}\mspace{14mu}{surface}\mspace{14mu}{area}} \right)}}$

The areas required for the above calculation may be determined byinspecting the surface of the porous support using a SEM. Thus, in apreferred embodiment, the porous support has a % porosity >1%, morepreferably >3%, especially >10%, more especially >20%.

The porosity of the porous support may also be expressed as a CO₂ gaspermeance (units are m³ (STP)/m²·s·kPa). When the composite membrane isintended for use in gas separation the porous support preferably has aCO₂ gas permeance of 5 to 150×10⁻⁵ m³ (STP)/m²·s·kPa, more preferably of5 to 100, most preferably of 7 to 70×10⁻⁵ m³ (STP)/m²·s·kPa.

Alternatively the porosity is characterised by measuring the N₂ gas flowrate through the porous support. Gas flow rate can be determined by anysuitable technique, for example using a Porolux™ 1000 device, availablefrom Porometer.com. Typically the Porolux™ 1000 is set at the maximumpressure (about 34 bar) and one measures the flow rate (L/min) of N₂ gasthrough the porous support under test. The N₂ flow rate through theporous support at a pressure of about 34 bar for an effective samplearea of 2.69 cm² (effective diameter of 18.5 mm) is preferably >1 L/min,more preferably >5 L/min, especially >10 L/min, more especially >25L/min. The higher of these flow rates are preferred because this reducesthe likelihood of the gas flux of the resultant composite membrane beingreduced by the porous support.

The abovementioned % porosity and permeance refer to the porous supportused to make the composite membrane.

The porous support preferably has an average thickness of 20 to 500 μm,preferably 50 to 400 μm, especially 100 to 300 μm.

One may use an ultrafiltration membrane as the porous support, e.g. apolysulfone ultrafiltration membrane, cellulosic ultrafiltrationmembrane, polytetrafluoroethylene ultrafiltration membrane,polyvinylidenefluoride ultrafiltration membrane and especiallypolyacrylonitrile ultrafiltration membrane. Asymmetric ultrafiltrationmembranes may be used, including those comprising a porous polymermembrane (preferably of thickness 10 to 150 μm, more preferably 20 to100 μm) and optionally a woven or non-woven fabric support. The poroussupport is preferably as thin as possible, provided it retains thedesired structural strength.

Typically the discriminating layer is present on one side of the poroussupport or is partially or wholly within the porous support.

Preferred discriminating layers comprise a polyimide, especially apolyimide having —CF₃ groups. Polyimides comprising —CF₃ groups may beprepared by, for example, the general methods described in U.S. Pat.Reissue No. 30,351 (based on U.S. Pat. No. 3,899,309) U.S. Pat. No.4,717,394 and U.S. Pat. No. 5,085,676. Typically one or more aromaticdianhydrides, preferably having —CF₃ groups, are condensed with one ormore diamines. The diamine(s) and dianhydride(s) copolymerise to form anAB-type copolymer having alternating groups derived from the diamine(s)and dianhydride(s) respectively.

Preferably the discriminating layer comprises groups of the Formula (1)wherein Ar is an aromatic group and R is a carboxylic acid group, asulphonic acid group, a hydroxyl group, a thiol group, an epoxy group oran oxetane group:

Optionally there may be a polymeric layer between the porous support andthe discriminating layer, often referred to as a gutter layer. Preferredgutter layers comprise a dialkylsiloxane.

Preferably the membrane stack comprises a plurality of membraneenvelopes and permeate carriers, wherein the permeate carriers arelocated between the membrane envelopes. The permeate carriers preferablycomprise one or two permeate spacer sheets. When the permeate carriercomprises two permeate spacer sheets preferably it further comprises agas impermeable sheet located between the two permeate spacer sheets.The permeate spacer sheets typically have a relatively large mesh sizeto allow the permeate gas to travel to the permeate collection tube. Inmost instances, permeate carriers will be utilized, but it is possibleto construct a module without permeate carriers. In general, thepermeate spacer sheets are formed of any inert material which maintainsa space between the membrane envelopes. Further, the feed spacer (whichis part of the membrane envelope) allows the gas to be filtered (orseparated) to travel axially along the membrane module.

Preferred materials for the permeate spacer sheets and feed spacer areopen, channel forming grid materials, such as polymeric grid, orcorrugated or mesh materials. Preferred among these are polypropyleneand other polyolefin netting materials.

Typically the edges of adjacent membrane sheets which lie along theaxial length of the permeate collection tube are sealed so that gasflowing through feed spacer screen is prevented from reaching thepermeate collection tube without first passing through the membranes.Alternatively, the membrane sheet may be folded with the fold beingadjacent to the permeate collection tube and with the feed spacerlocated within the fold such that membrane surfaces face one another. Inthis case, only one membrane sheet is needed per membrane envelope.

The permeate carrier (when present) and the membrane envelopes arepreferably spiral wound around a permeate collection tube with thepermeate carrier in gas communication with the permeate collection tube.Referring to the series of layers of membrane sheet, feed spacer and asecond membrane sheet as a membrane envelope, typically a plurality ofmembrane envelopes are spiral wound about a permeate collection tubewith a permeate spacer (e.g. a screen) located between each envelope.

After the membrane envelopes and optional permeate spacers have beenwound around the permeate collection tube, the assembly may be held in awound state using restraining bands or outer wraps, or a combinationthereof. A preferred method for restraining the assembly is by filamentwinding, in which a glass fibre filament dipped in an epoxy resin iswound around the assembly and cured. The assemblies can then be loadedinto a housing or pressure vessel which may be operated at a slightpressure drop across the module as the gas being filtered flows through.In operation, the feed gas to be filtered is introduced at one end faceof the membrane module.

The feed gas travels axially along membrane module through the feedspacers in the membrane envelope(s). As the feed gas encounters themembrane walls, part of the feed gas (the permeate) passes through themembrane and into the space between the membrane envelopes, which spaceis preferably occupied by a permeate carrier. The permeate gas travelsalong the permeate carrier, eventually passing into the permeatecollection tube through the perforations. The permeate exits the modulethrough the permeate collection tube and the retentate travels axiallythrough the module along the feed spacer.

As will be appreciated, it is necessary to provide a gas-tight sealbetween adjacent membrane envelopes, with the exception of the edgeadjacent to the permeate collection tube, in order to prevent the feedgas from entering the permeate carrier without first passing through amembrane wall of the membrane envelope.

The membrane envelope stack according to a first aspect of the presentinvention may be prepared by any suitable method, for example by amethod comprising:

-   (a) providing a plurality of membrane envelopes;-   (b) stacking the plurality of membrane envelopes and bonding the    membrane envelopes together by means of a partially cured adhesive,    optionally also providing a permeate carrier between each membrane    envelope;    and-   (c) curing the partially cured adhesive to give a cured adhesive;    wherein the adhesive adheres the membrane envelopes together to give    a glue line laminate having a tensile E-modulus of at least 1600    N/mm² and/or an elongation at break of 13% or less.

The above process forms a third aspect of the present invention.

The present invention also provides a method for preparing a spiralwound gas filtration module comprising the steps:

-   (i) providing a permeate collection tube having perforations;-   (ii) providing a membrane envelope stack comprising membrane    envelopes bonded together by means of a partially cured adhesive;-   (iii) winding the membrane envelope stack comprising the partially    cured adhesive about the permeate collection tube; and-   (iv) curing the partially cured adhesive to give a cured adhesive;    wherein the adhesive adheres the membrane envelopes together to give    a glue line laminate having a tensile E-modulus of at least 1600    N/mm² and/or an elongation at break of 13% or less.

In the methods according to the present invention, the adhesive may beapplied as a glue line along (or near) three edges on the outside facesof the membrane envelopes, e.g. near to the edges of the permeatecarrier and/or the membrane sheet(s). In this way, three sides of eachmembrane envelope (typically two long sides and one short side) areadhered to the next membrane envelope in a gas-tight manner, with thefourth side (typically a short side) being open for gas communicationwith the permeate collection tube.

When partial curing or further curing of the adhesive (e.g. in step (c)or step (iv)) is performed by a process comprising heating the adhesive,such heating will generally be performed at a temperature which does notdamage any components of the membrane envelope. The membranes can insome cases be temperature sensitive and so high curing temperaturesshould generally be avoided, depending on the properties of themembrane(s).

Partial curing of the adhesive can be advantageous because it increasesviscosity of the adhesive, thereby making step (iii) easier and lessmessy. As the adhesive is only partially (not fully) cured, the variouslayers can still move relative to one another during winding step (iii).

The partial curing of the adhesive is also useful because some movementof the layers to be adhered together is desirable during the subsequentwinding step because the outer layers travel a further distance duringwinding than the inner layers. Thus the adhesive is preferably partiallycured such that the membrane sheets are able to move relative to eachother during winding step (iii).

The adhesive may be partially cured by heating or irradiating theadhesive. When partial cure is achieved by heating the adhesive, thetemperature of the adhesive is preferably increased by less than 15° C.,more preferably less than 5° C. or even not at all. Typically theadhesive is partially cured by a process comprising heating it to atemperature of at most 25° C.

Optionally the adhesive is partially cured by a process comprisingageing the adhesive for at least 30 minutes, preferably at least 60minutes, more preferably at least 90 minutes, before performing windingstep (iii).

The curing in step (c) or (iv) is typically performed by heating themembrane envelope to a temperature higher than that used to partiallycure the adhesive and/or for a longer period than that used to partiallycure the adhesive. For example, the heating to partially cure theadhesive preferably raises the temperature of the adhesive by at least5° C., more preferably at least 10° C., especially at least 15° C.Preferably the heating to cure or partially cure the adhesive is to atemperature below 70° C., more preferably below to a temperature below50° C. (especially when the membrane comprises a polyacrylonitrileporous support).

A wide range of chemical types of adhesives may be used in the presentmethod, including epoxy adhesives and/or polyurethane adhesives,provided that when the adhesive is cured the glue line laminate has atensile E-modulus of at least 1600 N/mm² and/or an elongation at breakof 13% or less. For multi-part adhesives, in order to achieve therequired tensile E-modulus or an elongation at break one will need toselect combinations of polymerisable substance and a hardening agent.

The adhesive is liquid when it is applied to one or more components ofthe membrane envelope. Preferably the adhesive is liquid at 20° C. Theviscosity of the adhesive is not particularly limited and highly viscousliquid adhesives may be used, provided that they do not unduly hinderthe winding in step (iii).

The E-modulus (also called the modulus of elasticity) may be measuredusing a tensile testing machine, for example by ASTM method D638.

Preferred glue line laminates have a tensile strength at break of atleast 250 N. The tensile strength at break may also be measured by ASTMmethod D638.

Preferred glue line laminates have an elongation at break of 10% orless, preferably 5% or less. The elongation at break may also bemeasured by the method of ISO 527-3:1995.

Preferably the adhesive when cured has a Tg 50° C., more preferably atleast 55° C., especially at least 60° C., more especially at least 65°C., particularly at least 70° C. The Tg may be measured by differentialscanning calorimetry using the method described in ISO-11357-2.

The abovementioned tensile E-modulus, elongation at break and Tg referto glue line laminate when the adhesive has cured.

The finding that the above elongation at break can be advantageous issurprising, particularly in view of the contrary teaching in '517 whichstates that it is essential for the adhesive to remain flexible whencured.

In a preferred embodiment the glue line laminate has a tensile E-modulusof at least 1600 N/mm², a maximum tensile strength at break of at least250 N and an elongation at break of 13% or less.

Preferably the glue line laminate has a tensile E-modulus of at least1750 N/mm², more preferably more than 2250 N/mm² and especially morethan 3000 N/mm².

Preferably the glue line laminate has an elongation at break smallerthan 13%, more preferably 10% or less, and especially 5% or less.

Preferred adhesives are multi-part adhesives, typically comprising apolymerisable substance and a hardening agent. When the two parts aremixed, typically shortly before use, an adhesive is formed by, forexample, a crosslinking reaction. Conveniently one may choosecombinations of the parts which cure faster in step (c) or (iv) (whenheated) than in step (b) or (iii), allowing a sufficient open-time forthe winding to proceed in step (iii).

Preferred epoxy adhesives comprise, as polymerisable substance, an epoxyresin. Suitable epoxy resins are based on a bisphenol, e.g. bisphenol A,bisphenol F, bisphenol AF, bisphenol S; fluorinated epoxy resins;polyglycidyl ethers of polyglycols; polyglycidyl esters ofpolycarboxylic acids; cycloaliphatic epoxy resins; epoxy phenol novolacresins; epoxy cresol novolac resins; glycidylamine epoxy resins;tris(glycidyloxyphenyl)methane; tetrakis(glycidyloxyphenyl)ethane andtetraglycidyl diaminodiphenylmethane; and mixtures comprising two ormore of the foregoing.

Preferred hardening agents include polyamines (e.g. aliphatic,cycloaliphatic, aromatic and heterocyclic polyamines andpolyaminoamides); anhydrides (e.g. aliphatic and cycloaliphaticanhydrides); polyphenols (e.g. polyphenylene; dicyanodiamide andpolythiols.

The adhesives optionally comprise further additives, e.g. a retarder,plasticizer, diluent, rheology adjuvant, thixotropy-conferring agent,thickener, wetting agent, adhesion promoter, aging protection agentand/or stabilizer.

Preferably the membranes have a CO₂/CH₄ selectivity (αCO₂/CH₄) >10.Preferably the selectivity is determined by a process comprisingexposing the membrane to a 13/87 mixture by volume of CO₂ and CH₄ at afeed pressure of 6000 kPa at 40° C.

The method of the present invention is particularly useful for preparingspiral wound gas filtration modules for separating a gas into aretentate portion and a permeate portion, comprising a membrane envelopestack according to the first aspect of the present invention, optionallyone or more permeate carriers, and a permeate collection tube havingperforations along its length, said envelope stack and permeate carriers(when present) being wound around the permeate collection tube.

While this specification emphasises the usefulness of the membraneenvelope stacks and modules of the present invention for separatinggases, especially polar and non-polar gases, it will be understood thatthe they can also be used for other purposes, for example providing areducing gas for the direct reduction of iron ore in the steelproduction industry, dehydration of organic solvents (e.g. ethanoldehydration), pervaporation, oxygen enrichment, solvent resistantnanofiltration and vapour separation.

The membrane envelope stacks and modules of the invention may be used inconjunction with other gas separation techniques if desired, e.g. withsolvent absorption (e.g. Selexol, Rectisol, Sulfinol, Benfield), amineabsorption (e.g. DEA, MDEA), physical adsorption, e.g. pressure swingadsorption, cryogenic techniques, etc.

The membrane envelope stacks and modules according to the invention areparticularly useful for the separation of a feed gas (which includes afeed vapour) containing a target gas into a gas stream rich in thetarget gas and a gas stream depleted in the target gas. For example, afeed gas comprising polar and non-polar gases may be separated into agas stream rich in polar gases and a gas stream depleted in polar gases.In many cases the membranes have a high permeability to polar gases,e.g. CO₂, H₂S, NH₃, SO_(x), and nitrogen oxides, especially NO_(x),relative to non-polar gases, e.g. alkanes, H₂, N₂, and water vapour.

The target gas may be, for example, a gas which has value to the user ofthe membrane and which the user wishes to collect. Alternatively thetarget gas may be an undesirable gas, e.g. a pollutant or ‘greenhousegas’, which the user wishes to separate from a gas stream in order tomeet product specification or to protect the environment.

The membrane envelope stacks and modules according to the invention areparticularly useful for purifying natural gas (a mixture whichpredominantly comprises methane) by removing polar gases (CO₂, H₂S); forpurifying synthesis gas; and for removing CO₂ from hydrogen and fromflue gases. Flue gases typically arise from fireplaces, ovens, furnaces,boilers, combustion engines and power plants. The composition of fluegases depend on what is being burned, but usually they contain mostlynitrogen (typically more than two-thirds) derived from air, carbondioxide (CO₂) derived from combustion and water vapour as well asoxygen. Flue gases also contain a small percentage of pollutants such asparticulate matter, carbon monoxide, nitrogen oxides and sulphur oxides.Recently the separation and capture of CO₂ has attracted attention inrelation to environmental issues (global warming).

The membrane envelope stacks and modules according to the invention areparticularly useful for separating the following: a feed gas comprisingCO₂ and N₂ into a gas stream richer in CO₂ than the feed gas and a gasstream poorer in CO₂ than the feed gas; a feed gas comprising CO₂ andCH₄ into a gas stream richer in CO₂ than the feed gas and a gas streampoorer in CO₂ than the feed gas; a feed gas comprising CO₂ and H₂ into agas stream richer in CO₂ than the feed gas and a gas stream poorer inCO₂ than the feed gas, a feed gas comprising H₂S and CH₄ into a gasstream richer in H₂S than the feed gas and a gas stream poorer in H₂Sthan the feed gas; and a feed gas comprising H₂S and H₂ into a gasstream richer in H₂S than the feed gas and a gas stream poorer in H₂Sthan the feed gas.

EXAMPLES Preparation of the Adhesives

Adhesives were prepared by mixing the polymeric substances and thehardening agents mentioned in Table 1 at room temperature, in a weightratio of 1:1. The polymeric substances further comprised 3.5 wt % offumed silica.

The Aradur, Fermadur, Epikure and Ancamide, 2216a and UR3543a hardeningagents were obtained from Huntsman, Sonderhoff, Momentive, Air Products,3M respectively The Araldite polymerisable substance (epoxy andpolyurethane adhesives) were obtained from Huntsman. Other polyurethaneadhesives used were LA115-13-5 from Sonderhoff and UR3543 from HBFuller.

Further an epoxy resin adhesive 2216 polymerisable substance was used(obtained from 3M).

Measurement of Tensile E-Modulus, Tensile Strength at Break andElongation at Break

The tensile E-Modulus, tensile strength at break and elongation at breakmeasurements were performed on test strips having the dimensions shownin FIG. 3 cut from the glue line laminate. These properties weremeasured using a Zwick Z010 4 millimeter as per the method of ISO527-3:1995 at 22° C., relative humidity of 50% and using a crossheadspeed of 30 millimeters/minute. The results are shown in Table 1.

Assessment of Blister-Formation

The level of blister formation was measured on simulated glue linelaminates as follows:

Identical first and second membranes were prepared by applying adialkylsiloxane gutter layer to a polyacrylonitrile porous support,followed by the application of a polyimide discriminating layer. Themembranes had a thickness of <1.5 μm, as judged by scanning electronmicroscopy.

As permeate carrier there was used an epoxy-coated, warp knitted tricotfabric of thickness 0.33 mm and weight of 105-117 g/m², coated withepoxy.

The simulated glue line laminates were prepared as follows. Theadhesives mentioned in Table 1 under test (2 cm³) were applied sample ofthe first membrane sheet (50 mm×100 mm dimension). A permeate carrierconsisting of two permeate spacer sheets of similar dimensions wasapplied to the side of the first membrane carrying the adhesive. Asample of the second membrane sheet (50 mm×100 mm dimension) was thenplaced on top to match its orientation, such that the permeate carrierwas sandwiched between the two membrane sheets and such that theadhesive penetrated through the permeate carrier so that the permeatecarrier and both membrane sheets were in contact with the adhesive andcould be cured as a whole. A weight of 1 kg was applied on top and theadhesive was partially cured (for at least 16 hrs at room temperature).The adhesive was then further cured by heating at 50° C. for 16 hours inan oven to give a simulated glue line laminate.

The simulated glue line laminate was then inflated to a pressure of6,000 kPa with a simulated natural gas containing 13% CO₂, 87% CH₄ andtraces of toluene and exposing the inflated simulated stack to atemperature of 50° C. for 16 hours. The simulated natural gas wasreleased from the simulated stack and the stack was allowed to cool toroom temperature, then examined for the formation of blisters. Theresult of these tests is shown in the final column of Table 1 below.

TABLE 1 Tensile Elongation Adhesive Emodulus at break Blister ExamplePolymerisable Substance Hardening Agent (N/mm²) (%) level 1 Araldite ™GY250 Aradur ™ 140 3403 3 + 2 3M 2216 base Aradur ™ 140 3204 3 + 3Araldite ™ GY250 Ancamide ™ 2445 2325 9 + CEx1 3M 2216 base 3M 2216a1419 15 − CEx2 3M 2216 base Aradur ™ 3275 1427 16 −− CEx3 Araldite ™GY250 3M 2216a 1361 15 − CEx4 Sonderhoff LA115-13-5 SonderhoffFermadur ™ B-N 1237 15 −− CEx15 Araldite ™ XD4465 XD4782 1433 15 −−CEx16 HBFuller UR3543 HBFuller UR3543 a 1278 14 −− (blister formation: += no blister, − = few to moderate amount of blisters, −− = manyblisters)

The invention claimed is:
 1. A membrane envelope stack for separatinggases comprising membrane envelopes bonded together by means of a curedadhesive to give a glue line laminate having a tensile E-modulus of atleast 1600 N/mm² and an elongation at break of 13% or less; and whereinthe tensile E-modulus and elongation at break are as measured on teststrips cut from the glue line laminate, the measurements being performedusing a Zwick Z010 4 millimeter as per the method of ISO 527-3:1995 at22° C., relative humidity of 50% and using a crosshead speed of 30millimeters/minute.
 2. The membrane envelope stack according to claim 1wherein the adhesive has a Tg of at least 50° C. when cured.
 3. Themembrane envelope stack according to claim 1 wherein the membraneenvelopes comprise a feed spacer sandwiched between two membrane sheets,or one membrane sheet is folded around the feed spacer.
 4. The membraneenvelope stack according to claim 3 wherein the membrane sheet(s) arecomposite membranes comprising a discriminating layer and a poroussupport.
 5. The membrane envelope stack according to claim 4 wherein thediscriminating layers comprise a polyimide having —CF₃ groups.
 6. Themembrane envelope stack according to claim 4 wherein the discriminatinglayer comprises groups of the Formula (1) wherein Ar is an aromaticgroup and R is a carboxylic acid group, a sulphonic acid group, ahydroxyl group, a thiol group, an epoxy group or an oxetane group:


7. The membrane envelope stack according to claim 1 wherein the adhesiveis a multi-part adhesive comprising a polymerisable substance and ahardening agent.
 8. The membrane envelope stack according to claim 1wherein the glue line laminate comprises of two membranes and anadhesive.
 9. The membrane envelope stack according claim 8 wherein theglue line laminate further comprises a permeate carrier.
 10. Themembrane envelope stack according claim 9 wherein the permeate carriercomprises one or two permeate spacer layers and optionally a gasimpermeable sheet located between the two permeate spacer layers.
 11. Aspiral wound gas filtration module comprising a membrane envelope stackaccording to claim
 1. 12. A method for preparing a membrane envelopestack according to claim 1 comprising: (a) providing a plurality ofmembrane envelopes; (b) stacking the plurality of membrane envelopes andbonding the membrane envelopes together by means of a partially curedadhesive, optionally also providing a permeate carrier between eachmembrane envelope; and (c) curing the partially cured adhesive to give acured adhesive; wherein the cured adhesive adheres the membraneenvelopes together to give a glue line laminate having a tensileE-modulus of at least 1600 N/mm² and an elongation at break of 13% orless; and wherein the tensile E-modulus and elongation at break are asmeasured on test strips cut from the glue line laminate, themeasurements being performed using a Zwick Z010 4 millimeter as per themethod of ISO 527-3:1995 at 22° C., relative humidity of 50% and using acrosshead speed of 30 millimeters/minute.
 13. A process for separationof a feed gas containing a target gas into a gas stream rich in thetarget gas and a gas stream depleted in the target gas comprisingpassing the feed gas through a module according to claim
 11. 14. Themembrane envelope stack according to claim 3 wherein the membraneenvelopes have a CO₂/CH₄ selectivity >10.
 15. The membrane envelopestack according to claim 14 wherein the adhesive is a cured adhesive andthe glue line laminate has a tensile E-modulus of at least 1600 N/mm²and an elongation at break of 13% or less and the adhesive has a Tg ofat least 50° C. when cured; and wherein the tensile E-modulus andelongation at break are as measured on test strips cut from the glueline laminate, the measurements being performed using a Zwick Z010 4millimeter as per the method of ISO 527-3:1995 at 22° C., relativehumidity of 50% and using a crosshead speed of 30 millimeters/minute.